This invention relates to the fusing of toner images in electrographic apparatus. More particularly, this invention relates to a roller fuser which selectively fuses copy sheets carrying simplex or duplex toner images and to the control of the application of fuser release material to such roller fuser in order to maintain the amount of fuser release material applied to the fuser at a optimal rate per copy sheet.
In the electrophotographic process, toner images corresponding to the images of original documents are sequentially formed on a photoconductor and transferred to copy sheets to which the images are permanently fixed such as by the application of heat and/or pressure. In the production of a simplex copy, a toner image is formed on only one side of a copy sheet, whereas in the production of a duplex copy, toner images are formed on both sides of a copy sheet. In either case, one technique for permanently fusing the toner image(s) to the copy sheet is by passing the sheet through the nip of a pair of fuser rollers either one or both of which are heated and which are in pressure engagement. Since there is a tendency for toner to offset onto a heated roller, it has been customary to form the fusing layer of one or both of the fuser rollers with a material which is highly resistant to offsetting or sticking of toner. Typical of such materials are those which are resistant to degradation at high fusing temperatures such as polytetrafluoroethylene resin, silicone elastomer, flurosilicone elastomer, and the elastomer comprising a copolymer of tetra-fluoroethylene and polypropylene.
In order to increase the efficiency of fusing layers which are highly resistant to toner offset, it has been customary to apply fuser release material such as fuser oil to the fusing layer surface as it comes into contact with copy sheets having toner images to be fused. Ideally, the amount of fuser oil applied to the fuser rollers should be maintained at an optimum level over the entire surface of the rollers and for the duration of a copy run. However, in practice, this has not always been easy to accomplish. Thus, if elastomers are used as the fusing layer, the application of fuser oil of a similar composition as the elastomer (e.g., the application of silicone oil to a silicone elastomer), causes the elastomeric material to swell and to degrade over a period of time. If too little fuser oil is applied to the fuser roller surface, the surface tends to become contaminated with toner and other debris. On the other hand, if too much fuser oil is applied to the surface of the fuser roller, too much oil will be deposited on the fused copy sheets, causing undesirable staining of the sheets. Since a certain amount of fuser oil will be removed by each copy sheet processed by the roller fuser, a difficulty arises when simplex and duplex copies are randomly processed by the roller fuser. Since in the simplex copy mode twice as many copy sheets will be processed by the roller fuser than in the duplex copy mode, if the rate of fuser oil application to the fuser rollers is not adjusted for this change in copy mode, then undesirable under- or over-application of fuser oil to the roller fuser will result.
Heretofore, many techniques for controlling the rate of application of fuser oil to a roller fuser have utilized either a predetermined quantity of oil to be applied per unit of time or a predetermined quantity of oil to be applied per image area or frame count of a photoconductor. Such techniques have a number of disadvantages. Where fuser oil is added as a function of the count of number of image frames of a photocoductor which pass a sensor, difficulties arise if the frame count does not equal the copy count because of extra frames which are counted at the beginning and the end of a copy run. If the copier is run in a single pass duplex mode such as disclosed in U.S. Pat. No. 4,095,979, there will be twice as many image frames counted as the number of copies exiting from the copier thus causing an excess of fuser oil to be applied. Moreover, in certain types of copiers using a recirculating feeder, the time for recirculating only two documents to be copied in a collate mode may be greater than the time for the passage of two frames of the photoconductor, thus causing inaccuracies in fuser oil applications. Where fuser oil is applied as a function of time, difficulties may arise due to synchronization inaccuracies between real time clocks and copy count caused by jams in the copiers at the end of runs, by variations in copy throughput because of the selection of simplex or duplex mode and by voltage variations in the copier electrical system.